4.9. Serum Stability Assay

Peptide stability was evaluated in mouse serum as previously described [19,20]. In brief, formulated ¹⁸F-radiolabeled peptide (~3.7 MBq in PBS) was incubated with mouse serum (500 μL, Sigma Aldrich, St. Louis, MO) at 37 °C for 1 h. An aliquot of ¹⁸F-radiolabeled peptide in mouse serum (100 μL) was taken at 1 h, mixed with absolute ethanol (500 μL, 4 °C), and centrifuged to precipitate serum protein. The supernatant was diluted with HPLC solvent A before radio-HPLC analysis to determine the fraction of intact ¹⁸F-radiolabeled peptide.